1. Field of the Invention
The invention relates to piezoelectric microcantilever sensors for biosensing. More specifically, the invention relates to highly sensitive piezoelectric microcantilevers capable of determining the presence and/or mass of organic compounds. Applicable fields of use may include biodefense, food safety and pathogen detection.
2. Description of the Related Technology
Current biosensing technologies utilize quartz crystal microbalances (QCM), silicon microcantilevers, electrochemical enzyme immunoassays, fluorescence, laser-based or fiber-optics-based methods, amplification schemes such as polymerase chain reaction (PCR), or bound metal particles to determine the presence and/or mass of organic compounds. These techniques, however, fail to provide quantitative, efficient or highly sensitive detection. In addition to lacking sensitivity, they are also incapable, in some cases, of simultaneously monitoring multiple compounds or being used in high throughput array applications.
TABLE 1Comparison of PEMS with Other BiosensorsDetectionLabel-Direct,High-Detector TypesensitivityfreeIn-situRapidthroughputMultiplexingPEMS10−15-10−18gyesyesyesyesyesQCM10−9gyesyesYesNonoSilicon10−12gyesnoNoNonomicrocantileverOptical fibernonoNoYesyesfluorescenceELISA2610−10gnonoNoYesyesSPR310−12gyesyesYesNono
Of these technologies, QCM, which utilizes thickness-mode resonance sensing, is one of the most common commercially available biosensing technologies. Detection sensitivity of a QCM is related to the resonance frequency and the thickness of the quartz membrane. A resonance frequency of about 5 MHz, corresponding to a quartz membrane thickness of 330 μm, enables a minimum detectable mass density of about 10−9 g/cm2. Sensitivity is therefore generally limited to a range of about 10−8 g/Hz.
To increase sensitivity, some biosensors utilize silicon-based microcantilevers, which offer a sensitivity of approximately 10−12 g/Hz, about three orders of magnitude higher than QCMs. Advantageously, silicon microcantilevers are also widely available and may be easily integrated with existing silicon fabrication methodologies. Most silicon microcantilevers, however, rely on complex external optical components for deflection detection, an external driving mechanism for actuation and also require laser alignment. Moreover, because they are not piezoelectric, silicon microcantilevers are inferior for in-solution sensing, yielding low resonance peaks upon immersion in a solution.
Piezoelectric cantilevers, in comparison, use electrical means for detection and are not encumbered by the complexity and mass of the silicon-based sensors. Constructed from lead zirconate titanate (PZT), they are capable of electrical self-excitation and self-sensing for in-situ electrical detection. Currently, piezoelectric biosensors are millimeter-size cantilevers made by bonding commercial PZT to non-piezoelectric substrates such as stainless steel, titanium or glass. Although capable of in-situ biosensing, these millimeter-size cantilevers lack the desired sensitivity for such applications.
Thin-film-based PZT microcantilevers, such as those disclosed in JP-07027559 A2 and U.S. Pat. No. 7,084,554, are highly sensitive instruments. U.S. Pat. No. 7,084,554, in particular, discloses a thin piezoelectric film biomorph capable of being formed as a cantilever. The biomorph may be composed of a piezoelectric thin PZT film of about 1-10 μm in thickness for the purpose of increasing the working frequency range of micro-electro-mechanical dimensioned (MEMS) systems. The patent further teaches that the piezoelectric thin film may be fabricated by thin film fabrication methods such as a sol-gel method, sputtering, hydrothermal methods, chemical vapor deposition (CVD) or another thin film fabrication method, followed by low temperature annealing and dry etching, plasma etching or patterning by wet chemical etching (See col. 5, lines 55-64 of U.S. Pat. No. 7,084,554). These piezoelectric microcantilevers, however, are incapable of in-situ electrical detection due to degradation of resonance peaks in solution.
Recent advances in thin-film PZT microcantilevers incorporate an electrical insulation layer that prevents liquid damping. U.S. Patent Publication no. 2005/0112621 discloses an insulation layer surrounding a PZT microcantilever having a thin piezoelectric film in order to prevent conduction in liquid media (See e.g. col. 4, lines 28-36).
However, there remains a need for a piezoelectric microcantilever device capable of in-solution biologic detection having a femtogram or higher sensitivity. In addition, there remains a need for a device consisting of array piezoelectric microcantilevers that is capable of simultaneous detection of multiple compounds.
Advancement in the sensitivity, accuracy and efficiency of electrical biosensing is essential to the developing field of bioterrorism defense. During the fall of 2001, for example, bacillus Anthracis spores, a bioterrorism agent, were responsible for the deaths of 5 individuals and infection of 17 others. As bioterrorism threats become more prevalent, there is a growing need for reliable in-situ detectors capable of efficiently detecting multiple biological agents in real time.
Advancements in biosensing accuracy, sensitivity, multi compound detection are also potentially useful in the health sciences for early detection and prevention of diseases. Breast cancer, for example, is the second leading cause of death for women. Although a number of potential breast cancer markers, such as HER2 (HER2/neu, c-erbB-2), EGFR, CA-15-3, CA27.29, urokinase plasminogen activator receptor (uPAR), carcinoembryonic antigen (CEA), α-fetoprotein (AFP), and cytokeratins, are known, no blood test or method for detecting these markers currently exists. Mammography is frequently inadequate and often produces false positives leading to unnecessary biopsies. Therefore early detection methods for various cancers and other diseases lacking adequate diagnostic means, like breast cancer, that are capable of accurately, effectively and non-invasively identifying and quantifying pathogens and other disease markers are also needed.